In vehicles having conventional manual transmissions, when moving away from rest, some skill is required from the driver to maintain suitable engine speed while applying useful motive power to the wheels. If the clutch is released too quickly, the engine can be stalled, requiring a re-start. Other error states can include noisy over-revving of the engine, rough and jerky motion, or failure to achieve the desired vehicle position or rate of acceleration. These problems can generally be overcome with skill and experience, but they dissatisfy less skilled drivers and those encountering a different vehicle for the first time.
A second problem arises in that rapid clutch engagement on pull-away, engine bump start or bad gear changing, result in excessive stresses on transmission, driveline and suspension components. These can therefore be made cheaper and lighter if this effect can be reduced.
The root cause is that for a short time after the clutch is released, the speeds of the drivetrain and wheels are not yet synchronised with the engine. The full “slip torque” of the friction clutch, with its full clamp pressure, is transmitted through the above components, regardless of engine torque or tyre friction. This slip torque is typically from 1.3 to 3 times the highest output torque that the engine can deliver in steady state operation. The clutch slip torque and the gear ratios etc. are the only significant factors controlling the resulting peak torque, unless the actuation system can slow the engagement of the clutch.
These problems are worsened by current trends towards higher clutch slip torque, making the error states more difficult to avoid.
A known attempt at mitigating the above problem employs a non-return valve that allows free flow of fluid from the clutch master cylinder to the clutch slave cylinder. This flow causes the clutch to be disengaged. A small orifice is combined in the same device, to allow return flow of fluid from the slave cylinder to the master cylinder, but at a slow rate, when the clutch pedal is released to re-engage the clutch. This flow is deliberately slowed to slow the engagement of the clutch, reducing the peak torques arising on clutch engagement.
Typically the orifice is required to make little or no difference to normal driving events, but only to restrict the fierceness of unusually sudden engagements. However these effects are difficult to achieve at both low and high temperatures.
The above solution can increase the clutch pedal return time to an unacceptable degree. The controllability is most severely affected in cold temperatures, owing to raised viscosity of the fluid. These limiters can therefore only provide limited control, which does not provide an acceptable solution to the main problem, and only limited relief for the second highlighted problem.